C.D.C. Urges Zika Testing for Some Who Are Pregnant

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A researcher at São Paulo University in Brazil holds a container of Aedes aegypti mosquitoes, which can spread the Zika virus. On Tuesday, the C.D.C. issued guidelines for pregnant women who have visited countries where the virus is spreading.CreditAndre Penner/Associated Press

Pregnant women who feel sick and have visited countries in which the Zika virus is spreading should see a doctor soon and be tested for infection even though the tests are imperfect, federal health officials said on Tuesday.

That advice was at the core of interim Zika-related guidelines for pregnant women issued by the Centers for Disease Control and Prevention. The authors are specialists in emerging diseases and reproductive health.

The guidelines included a “testing algorithm” to show doctors how to proceed with a worried patient who is pregnant and has recently lived in or traveled to an area where the virus is being transmitted.

Infection with the virus has been linked to brain damage and microcephaly — unusually small heads in infants.

For such a woman, the picture the guidelines presented is “pretty bleak,” said Dr. William Schaffner, the chairman of preventive medicine at Vanderbilt University.

Some of the recommended tests result in false positive outcomes, he noted, while others are not useful until late in pregnancy. Moreover, the nation’s top laboratories simply don’t yet have the capacity to test all the women who should be tested.

Pregnant women who fell ill with Zika symptoms — these include fever, rash, joint pain and red eyes — during travel or two weeks afterward should get blood tests for the virus, the guidelines say. Those with no symptoms do not need blood tests.

That may cause controversy, experts said, because 80 percent of infected people never develop symptoms, and it is not known whether an asymptomatic infection can affect a fetus.

“That had me scratching my head,” Dr. Schaffner said. “Most cases are asymptomatic, and nothing I’ve read says that women need to be symptomatic for the baby to be affected.”

Dr. Denise J. Jamieson, one of the C.D.C. authors, acknowledged that there is no way to know whether the babies of mothers who did not get noticeably ill might be harmed.

The C.D.C does not recommend blood tests for asymptomatic women, Dr. Jamieson said, partly because the country’s laboratories simply cannot do that many tests right now.

The complex tests currently can be done only by the C.D.C. and a few state laboratories, and they would be overwhelmed by testing, for example, every pregnant woman living in Puerto Rico and every pregnant woman from the rest of the country who visited Latin America or the Caribbean in the last nine months.

“These are interim guidelines, and as soon we know more about the virus, they may evolve,” Dr. Jamieson said.

Zika also spreads in countries in Africa and Asia, but transmission is far less intense because the virus has been there for decades and many people have immunity. Islands in the South Pacific have experienced more recent outbreaks.

Extensive footnotes to the new guidelines note some serious drawback to the tests. For instance, blood testing for the virus itself only is possible within two weeks of the infection.

Tests for antibodies to the virus can be administered later, but may give false positives or inconclusive results if the patient has also been infected with dengue or other related viruses.

Dengue has widely circulated in Puerto Rico, the Caribbean and Latin America, and briefly even in Florida and Texas.

Whether a symptomatic woman’s test is positive or negative, the guidelines say, she should be offered a fetal ultrasound to look for microcephaly or calcification inside the developing skull.

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The C.D.C. testing algorithm for pregnant women who have visited countries in which the Zika virus is spreading.CreditCenters for Disease Control and Prevention

Unfortunately, microcephaly is generally not detectable by ultrasound before the end of the second trimester.

Women who choose to abort a microcephalic fetus would be forced to seek a late-term abortion.

Those can be performed with little medical risk to the mother, but can be emotionally wrenching, especially if a woman has been visibly pregnant for weeks.

If microcephaly is not present, the guidelines say, the doctor and patient should consider doing a series of ultrasound scans as the pregnancy develops to watch for any appearance of microcephaly.

Women who experienced symptoms and have a positive or inconclusive Zika test should be offered both an ultrasound and amniocentesis.

During amniocentesis, a needle is inserted and some of the amniotic fluid surrounding the baby is withdrawn and tested for the virus. A positive result suggests the fetus has been infected.

Mothers whose fetuses show any signs of microcephaly or skull hardening should also be offered amniocentesis, the guidelines said. The procedure, however, is not recommended until after a fetus is 15 weeks old. It is not known how long the virus might persist in a fetus.

Dr. Schaffner predicted that the ability of state and local laboratories to do more testing would improve as the C.D.C. speeds up distribution of test kits and training, which has been done effectively in past epidemics, including Ebola.

Many ultrasounds would be done on asymptomatic women, he also said, since plenty of those machines and technicians are available and doctors can offer concerned patients little else right now.

Dr. Schaffner said that obstetricians’ offices would be filled with worried women, and that their doctors — normally not obligated to think as much about exotic foreign viruses as infectious disease specialists are — would face a steep learning curve.

“This is a virus with a strange name, an international origin and potentially horrendous consequences,” he said. “This is going to be enormously anxiety-provoking.”

A version of this article appears in print on , on Page A3 of the New York edition with the headline: C.D.C. Urges Zika Testing for Some Who Are Pregnant. Order Reprints | Today’s Paper | Subscribe